1. Field of the Invention
This invention is directed to a method of operating in the load commutated mode an a-c drive comprising parallel connected polyphase d-c link power converters. More particularly, it is directed to firing load commutated controlled rectifier switches in polyphase output inverters of the power converters in a sequence which generates from constant d-c link currents in the power converters, polyphase, alternating load currents from each inverter which are phase shifted relative to one another by an amount which reduces selected harmonics in the composite load current.
2. Background Information
Variable speed a-c drives of the load commutated inverter (LCI) type utilize a d-c link power converter to convert line current of a given frequency to a load current of a controlled, variable frequency. Such a power converter includes an inverter to generate the load current of variable frequency from the d-c current generated by a line side rectifier. Typically, these inverters employ thyristors to gate portions of the d-c current to the load. Switches such as thyristors can not be turned off from the control electrode, but must be commutated by other means.
The LCI type drive is used with an over excited synchronous motor which presents a leading power factor to the inverter thyristors to allow the thyristors to operate as naturally commutated switches. In other words, the motor load commutates the switches. At standstill or very low speeds, however, the motor voltages are insufficient to effect this commutation and other means must be utilized. A solution to this problem has been to operate the power converter in a "pulsed mode" where the d-c link current is periodically forced to zero by the line side converter to allow the motor side inverter thyristors to commutate. The pulse mode forces the current to be square in nature, with significant fifth and seventh harmonic components, which in turn give rise to strong torque pulsations at six times the line frequency and harmonics thereof for a three-phase drive.
U.S. Pat. No. 4,084,220 is directed to an a-c drive designed to reduce these torque pulsations at low fundamental load frequencies. It calls for two d-c link power converters connected in parallel. Shaped d-c pulses are generated in the respective d-c circuits of the two converters. The shaped d-c pulse signals are phase shifted such that the resultant current gated to the load by the two load side inverters of the converters more closely approximates a sine-wave than a square wave. This system requires that the repetition rate of the pulsed d-c signal be three times the fundamental frequency of the load current. As a result, it is only suitable for reducing torque pulsation at low frequencies. As frequency increases, the thyristors can not be switched rapidly enough to maintain a repetition rate of the pulsed d-c which is three times that of the fundamental load frequency.
Hence the dual power converters of U.S. Pat. No. 4,084,220, like systems with a single power converter, are operated in the load commutated mode with constant d-c link currents at higher load frequencies, typically above about 12 Hz or so. With constant d-c link currents in the converters, the load currents generated by the output inverters are square in nature with the attendant problems discussed above. In accordance with the teachings of U.S. Pat. No. 4,084,220, the switches of the output inverters of the dual converters are operated in parallel in the load commutated mode so that the same square waves and torque pulsations are generated with the dual converters as with a single power converter.
It is the primary object of the invention to provide a method for operating an a-c drive having multiple power converters connected in parallel in a load commutated mode with reduced torque pulsations.
It is another object of the invention to provide such a method which permits reduction of selected harmonics in the load current.
It is also an object of the invention to satisfy the above objects in a simple, cost effective manner.
It is an additional object of the invention to satisfy the above objects without sacrificing the advantageous low speed operation achieved by the dual converter arrangement.